Mars Express scuppers greenhouse hopes

If life exists on the surface of Mars, it has not encountered liquid water for several billion years, scientists say. It seems that the European Space Agency's orbiter, Mars Express, has found no evidence of the necessary greenhouse effect.

The result comes from Mars Express's OMEGA instrument, which analyses visible and infrared light from the planet to reveal the chemical composition of the surface.

Mars Express has previously taken photographs of deep channels that suggest icy glaciers moved across the planet's surface just millions of years ago.

"But the question is whether the past conditions on Mars allowed water to be in liquid form," explains Jean-Pierre Bibring, OMEGA's principal investigator, who is based at the Institute for Space Astrophysics in Orsay, France. The latest results are presented in a suite of papers, published in this week's Science1 -6.

Early loss

Terrestrial experience tells us that all life needs water, so scientists have been eagerly hunting for clues that might suggest Mars was once warmer and wetter at some point in its history of four and a half billion years.

Early Mars could have kept warm with a blanket of carbon dioxide that would have absorbed and re-radiated heat lost from the surface. Such a greenhouse effect would have raised the temperature enough for water ice to melt. "But everything points to the fact that Mars lost its atmosphere early on," says Bibring.

After a global survey, Mars Express has found none of the carbonate minerals that would form under a dense carbon-dioxide atmosphere. This makes it unlikely that the planet ever experienced significant greenhouse warming, contrary to some scientists' expectations. "In a sense, we've been barking up the wrong tree for 20 years," says planetary geophysicist Lionel Wilson of the University of Lancaster, UK.

NASA's Mars Exploration Rovers, Spirit and Opportunity, have studied rock formations and mineral deposits that prove liquid water once flowed over the surface of Mars. But the exact timing of these aquatic events has remained elusive.

Bibring says that liquid water may have been common on the planet in its first billion years: "But after that, if there was life, it disappeared or was blocked into niches."

Polar ice

OMEGA, which stands for Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (the Observatory for Mineralogy, Water, Ice, and Activity), also looked for sulphate minerals, which tend to form in water.

Large deposits of sulphates at the north pole, as well around regions known as the Valles Marineris and Terra Meridian, provide the most comprehensive record yet of water-driven activity over large swaths of Mars.

And the craft has finally given researchers concrete evidence that the frosty poles are dominated by water ice. "It's the first time we're really identifying and measuring that," says Bibring.

Astronomers knew that Mars possessed polar ice caps, and measurements by NASA's Mars Global Surveyor craft found that the caps were a mixture of water and carbon dioxide ices. But it now seems that the south pole has only a thin veneer of frozen carbon dioxide that covers huge deposits of water ice.

"It's very useful to have more or less a guaranteed confirmation that the bulk of the caps is ice," says Wilson.